In order to allow the extraction of extra-heavy oils or bitumen from the known incidences of oil sand or oil shale, their flowability must be significantly increased. This can be achieved by increasing the temperature of the incidence (reservoir). The increase in flowability can be achieved either by introducing solvents or thinners and/or by heating or fusion of the extra-heavy oil or bitumen, for which purpose heating is effected by means of pipe systems that are introduced through boreholes.
The most widespread and commonly used in-situ method for extraction bitumen or extra-heavy oil is the SAGD (Steam Assisted Gravity Drainage) method. In this case, steam (to which solvents may be added) is forced under high pressure through a pipe which runs horizontally within the layer. The heated fused bitumen or extra-heavy oil, once separated from the sand or rock, seeps down to a second pipe which is laid approximately 5 m deeper and via which the extraction of the liquefied bitumen or extra-heavy oil takes place, wherein the distance between injector and production pipe is dependent on the reservoir geometry.
The steam has to perform several tasks concurrently in this case, specifically the introduction of heat energy for the liquefaction, the separation from the sand, and the build-up of pressure in the reservoir, in order firstly to render the reservoir geo-mechanically permeable for bitumen transport (permeability), and secondly to allow the extraction of the bitumen without additional pumps.
The SAGD method starts by introducing steam through both pipes for e.g. three months, in order firstly to liquefy the bitumen in the space between the pipes as quickly as possible. This is followed by the introduction of steam through the upper pipe only, and the extraction through the lower pipe can commence.
The German patent application DE 10 2007 008 292 A1 already specifies that the SAGD method normally used for this purpose can be complemented by an inductive heating apparatus. Furthermore, the German patent application DE 10 2007 036 832 A1 describes an apparatus in which provision is made for parallel arrangements of inductors or electrodes, which are connected above ground to a converter.
The earlier German patent applications DE 10 2007 008 292 A1 and DE 10 2007 036 832 A1 therefore propose inductive heating of the deposit in addition to the introduction of steam. If applicable, resistive heating between two electrodes can also be effected in this case.
In the cited earlier patent applications, individual inductor pairs comprising forward and return conductors, or groups of inductor pairs in various geometric configurations, are subjected to current in order to heat the reservoir inductively. In this case, a constant distance between the inductors is assumed within the reservoir, resulting in a constant heating power along the inductors in the case of homogenous electrical conductivity distribution. In the description, the forward and return conductors are guided in close spatial proximity in the sections in which the capping is breached, in order to minimize the losses there.
As described in the earlier applications, variation of the heating power along the inductors can be effected specifically by sectional injection of electrolytes, thereby changing the impedance. This requires corresponding electrolyte injection apparatus, whose installation can be resource-intensive and costly.